Move changes made in #10 to OOP.

[Benunc]

The new OOP structure planned for 6.0 needs to incorporate the Addon nag added in #10. Ideally, this nag would be configurable to be extended for new addons as they are made available.

The text could be changed out based on new updates to Core, as well as the image.

For example, now the nag says "Add Premium Style to your Better Click To Tweet boxes!" in the header and "Choose from multiple options when styling your Better Click To Tweet boxes, with no code." with a bullet list of features, an image, and a link to the sales page. A way to make those features extendable/replicable for future addons would be great.